1. Field of the Invention
The present invention relates to an apparatus for incinerating waste materials such as waste tires, for example.
2. Description of the Prior Art
One known apparatus for incinerating waste materials such as waste tires is disclosed in Japanese Laid-Open Patent Publication No. 2-135280. Specifically, the disclosed incinerating apparatus comprises an apparatus for incinerating a waste material by processing the waste material through dry distillation and gasification.
The disclosed incinerating apparatus has a gasification furnace in which a waste material is placed. In the gasification furnace, a portion of the waste material is burned while the remainder of the waste material is subjected to dry distillation (i.e., thermally decomposed) with the heat of combustion, until finally the waste material is fully burned and ashed. A combustible gas produced when the waste material is thermally decomposed is introduced through a gas passage into an combustion furnace. In the combustion furnace, the supplied combustible gas is mixed with oxygen (air) and burned at a temperature high enough to reduce nitrogen oxides to a sufficiently small level. In this manner, the waste material is incinerated without causing environmental pollution problems which would otherwise be posed by the generation of nitrogen oxides.
The thermal energy generated when the combustible gas is burned is used as a heat source for a boiler or the like. The generated thermal energy is therefore effectively utilized in the process of incinerating the waste material.
More specifically, the combustion of a portion of the waste material and the dry distillation of the remainder thereof in the gasification furnace, and the combustion of the combustible gas in the combustion furnace are carried out as follows:
The combustion of a portion of the waste material and the dry distillation of the remainder thereof in the gasification furnace are started when the portion of the waste material is ignited after the waste material is charged into the gasification furnace. When the portion of the waste material starts being combusted, the remaining waste material starts being subjected to dry distillation by the heat of combustion, and the combustible ga starts being generated by the dry distillation. As the dry distillation process progresses, the combustible gas is generated in a progressively larger quantity. Oxygen required to ignite the waste material and oxygen required to burn the portion of the waste material immediately after it has been ignited are supplied from the air in the gasification furnace.
The combustible gas generated in the gasification furnace is delivered through the gas passage and admitted into the combustion furnace in which the combustible gas is mixed with oxygen and ignited by an igniter mounted in the combustion furnace. At this time, the temperature of the combustible gas as it is burned in the combustion furnace is detected by a temperature sensor disposed in the combustion furnace. The detected temperature rises as the amount of the combustible gas generated in the gasification furnace becomes larger and so does the the amount of the combustible gas introduced into the combustion furnace. When the combustible gas starts being combusted in the combustion furnace, an oxygen supply device connected to the gasification furnace operates to supply the gasification furnace with the oxygen that is needed to burn the portion of the waste material and thermally decompose the remaining waste material. The oxygen is supplied to the gasification furnace at a rate that is regulated to keep the detected temperature of the combustible gas at a predetermined, substantially constant level for reducing any nitrogen oxides to a sufficiently small amount.
More specifically, when the temperature of the combustible gas becomes lower than the predetermined, substantially constant temperature, the amount of oxygen supplied to the gasification furnace is increased to accelerate the combustion of the portion of the waste material and the dry distillation of the remainder of the waste material. As a result, the combustible gas is produced in an increased quantity, thereby increasing the temperature of the combustible gas in the combustion furnace.
Conversely, when the temperature of the combustible gas becomes higher than the predetermined, substantially constant temperature, the amount of oxygen supplied to the gasification furnace is reduced to suppress the combustion of the portion of the waste material and the dry distillation of the remainder of the waste material. As a consequence, the combustible gas is produced in a reduced quantity, thereby lowering the temperature of the combustible gas in the combustion furnace.
In this manner, the temperature of the combustible gas as it is burned in the combustion furnace is maintained at the substantially constant level for sufficiently reducing any nitrogen oxides. The combustible gas is substantially completely burned in the combustion furnace without causing an environmental pollution. At the same time, the combustion of the portion of the waste material and the dry distillation of the remainder of the waste material are allowed to progress smoothly in the gasification furnace. The thermal energy produced when the combustible gas is burned can be effectively utilized as a heat source for a boiler or the like. The partial combustion of the waste material in the gasification furnace is progressively shifted into a region of the waste material where the combustible gas has been generated, i.e., is carried out in a region of the waste material where it is thermally decomposed. Accordingly, the partial combustion takes place while producing almost no nitrogen oxides.
Upon the dry distillation of the waste material, the waste material is progressively ashed as the partial combustion of the waste material progresses. Therefore, the combusted portion of the waste material is progressively shifted into another region where the dry distillation is substantially finished, and the portion of the waste material which can be thermally decomposed by dry distillation is progressively reduced.
Eventually, the waste material in the gasification furnace becomes unable to produce the amount of combustible gas which is required to keep the temperature of the combustible gas at the substantially constant level in the combustion furnace. Thus, the temperature of the combustible gas as it is turned is lowered. At this stage, the waste material in the gasification furnace is burned in a substantial portion thereof except the portion that has already by ashed. Thereafter, the waste material is finally ashed by the combustion thereof.
If the waste material were burned while being subjected to an insufficient degree of dry distillation, then an increased amount of nitrogen oxides would be produced. Therefore, the waste material should preferably be burned after it has been subjected to as much dry distillation as possible. To burn the combustible gas while minimizing the generation of nitrogen oxides, it is preferable that the combustible gas be produced by dry distillation in order to keep itself at a substantially constant temperature to minimize nitrogen oxides. Furthermore, from the standpoint of better incineration of the waste material, the waste material should preferably be burned and ashed when the dry distillation thereof is smoothly and reliably completed.
To meet the above requirements, while the dry distillation of the waste material is in progress in the conventional incinerating apparatus, oxygen is supplied from the oxygen supply device to the gasification furnace to keep the temperature of the combustible gas at a substantially constant level for continuous and stable combustion of a portion of the waste material and also continuous and stable dry distillation of the remaining waste material, as described above. When the dry distillation is finished, the oxygen supply device supplies an increased amount of oxygen to the gasification furnace according to the temperature in the gasification furnace as detected by a temperature sensor therein for promoting final combustion and ashing of the waste material.
Specifically, while the dry distillation of the waste material progresses in a stable state, the temperature in the gasification furnace progressively goes up as the combustion of the portion of the waste material progresses, even though part of the heat of the combusted portion is absorbed by the thermally decomposed portion of the waste material. When the dry distillation is in a final stage, after the temperature sharply rises as described above, since the thermally decomposed portion of the waste material is reduced, and so is the heat of the combusted portion which is absorbed by the thermally decomposed portion, the temperature in the gasification furnace decreases as the combustion and ashing of the portion of the waste material progress.
In the conventional incinerating apparatus, when the temperature in the gasification furnace exceeds a predetermined temperature that has been selected experimentally and empirically, any portion of the waste material that can be thermally decomposed by dry distillation is regarded as being almost eliminated, and the amount of oxygen supplied to the gasification furnace is increased to accelerate the final combustion and ashing of the waste material.
However, because the amount of oxygen supplied to the gasification furnace is increased based on only the temperature in the gasification furnace upon final ashing of the waste material, the conventional incinerating apparatus has suffered from the following drawbacks:
As described above, in general, the temperature in the gasification furnace sharply rises and then drops in the final stage of dry distillation of the waste material. The temperature in the gasification furnace does not vary uniformly and smoothly, but is generally subjected to temporary fluctuations depending on various conditions, e.g., the type of the waste material being incinerated, the manner in which the waste material is placed, etc.
Even when a substantial portion of the waste material remains to be thermally decomposed by dry distillation, there may be temporary situations when the dry distillation does not take place smoothly depending on the type of waste material being incinerated, the manner in which the waste material is placed in the furnace, or other conditions. Consequently, the heat of the combusted portion of the waste material that is absorbed by the thermally decomposed portion thereof is temporarily reduced, with the result that the temperature in the gasification furnace may rise beyond the predetermined temperature which is used for determining the timing of final ashing of the waste material.
If this happens, regardless of the presence of a substantial portion of the waste material that can still be thermally decomposed by dry distillation, the amount of oxygen supplied to the gasification furnace is increased to accelerate the combustion and ashing of the waste material. Therefore, the waste material which has not been fully thermally decomposed yet is burned, producing an increased amount of nitrogen oxides and an unstable amount of combustible gas, which tends to produce a large amount of nitrogen oxides when burned.
Moreover, when the oxygen in the gasification furnace is present in an amount larger than necessary when the waste material is thermally decomposed by dry distillation, the combustible gas produced by the dry distillation of the waste material may be mixed with sufficient oxygen to burn the combustible gas, and the mixture may be introduced from the gasification furnace into the combustion furnace. In this case, the flames produced when the combustible gas is burned in the combustion furnace and the flames of the igniter are liable to be propagated from the combustion furnace through the gas passage into the gasification furnace, burning the combustible gas in the gasification furnace and the gas passage, a phenomenon known as a backfire. When such a backfire occurs, the temperature in the combustion and gasification furnaces greatly increases, tending to damage the combustion and gasification furnaces and various devices mounted therein.
The amount of oxygen in the gasification furnace is apt to become more than necessary particularly when the portion of the waste material is burned unstably, or when the combusted portion of the waste material is small, consuming a relatively small amount of oxygen, or when the combustible gas produced by the thermally decomposed portion of the waste material is comparatively small, or when external air flows into the gasification furnace because of poor insulation between the gasification furnace and the exterior space, during the initial stage of combustion and dry distillation of the waste material immediately after it has been ignited.
In the conventional incinerating apparatus, oxygen in the gasification furnace is utilized when the waste material is ignited and the portion of the waste material is burned right after the waste material has been ignited, and a single region of the waste material is ignited by a single igniter. Accordingly, the portion of the waste material is likely to be burned unstable in the initial stage, and the combusted portion of the waste material is small.
Even under such a condition, once the waste material is thermally decomposed by dry distillation and the produced combusted gas is ignited in the combustion furnace, the oxygen supply device supplies the gasification furnace with an amount of oxygen for keeping the temperature of the combustible gas at a substantially constant level. That is, the gasification furnace may be supplied with more oxygen than necessary while the combustion of the portion of the waste material is unstable and the oxygen combustion is small. For these reason, backfire is likely to happen in the conventional incinerating apparatus.